Coordination compounds are known for their photochemical
properties, which may result in photodissociation,
photosubstitution, photoisomerization, photoreduction, or photo-
oxidation, for example ( 3 ). In addition to inner-sphere
rearrangements, the complexes in excited states are susceptible
to transfer their energy or charges into other species, such as sol-
vent molecules, ion-pair partners, and other nonbonded
quenchers.
Photoreactions can change forms and the physicochemical
behavior of coordination compounds and thereby are responsible
for transition metal transport between the environmental com-
partments. Some of these photoreactions are also of significance
in natural cleaning: to ensure continuity of the processes, cata-
lyst reproducibility should be guaranteed, that is, the relevant
photoreactions should belong to the photocatalytic family.
III. Effect of Complexation and Photochemistry on Composition of
Individual Compartments and Transport Between ThemA. ROLE OFNATURALCHELATINGLIGANDS
One of the most important agents controlling metal transport
and their function in nature are chelating and macrocyclic lig-
ands. The former are ubiquitous in the environment coming from
both natural and anthropogenic sources. Their main natural
reservoirs are HS or natural organic matter, widespread existing
in water or soil environmental systems. These are heterogeneous
mixtures of a variety of organic compounds, consisting of aro-
matic, aliphatic, phenolic, and quinolic functional groups with
varying molecular sizes and properties. HS arise from decompo-
sition of plant and animal tissues in soils, waters, and sediments,
which cannot be classified as any other chemical class of com-
pounds. Humic materials consist of a skeleton of alkyl or aro-
matic units cross-linked mainly by oxygen and nitrogen groups
with the major functional groups being carboxylic acid, phenolic
and alcoholic hydroxyls, ketone, and quinone groups. Their size,
molecular weight, elemental composition, structure, number,
and positions of functional groups vary depending on the origin
and age of the material. According to their solubility, HS are
divided into fulvic acids, humic acids, and humins. The
structures of fulvic acids are somewhat more aliphatic and less
aromatic than humic acids, and fulvic acids are richer in carbox-
ylic, phenolic, and ketonic groups. This composition is
296 ZOFIA STASICKA